1. Field of the Invention The invention relates to an improved method of anisotropically etching semiconductor materials, wherein undercutting of silicon on insulator (SO) structures is substantially eliminated.
2. Description of the Related Art
An important step in the manufacture of silicon containing devices, which includes semiconductor chips, is the etching of different layers such as polysilicon and silicon which make up the finished semiconductor chip or thin film circuit.
SOI structures, such as trenches, have been shown to exhibit undercutting when etching down to the silicon-insulator interface.
The individual structures to be etched into the substrate are usually defined by the etching masks applied to the silicon substrate by way of so-called masking layers, for example, a photoresist layer, which after exposure to UV light and subsequent developing, remains on the substrate, thereby protecting the silicon layer from the etchant.
In an anisotropic etching technique, it is necessary to achieve a laterally exact defined recess (trench via contact) in the silicon. These deeply-extending recesses must have sidewalls which are to be as vertical as possible.
The edges of the masking layers covering those silicon substrate regions that are not supposed to be etched are not undercut in order to keep the lateral precision of the structural transition from the mask into the silicon as high as possible. As a result, it is necessary to allow the etching to progress only on the bottom of the structures, but not laterally on the already produced side walls of the structures.
To this end, it has been proposed to use a plasma-etching method to perform etching of profiles on silicon substrates. In this method chemically reactive species and electrically-charged particles (ions and electrons) are generated in a reactive gas mixture in a reactor with the aid of an electric discharge. The positively-charged cations generated in this manner are accelerated toward the substrate by means of an induced electrical bias through the application of an RF field to the silicon substrate, and fall virtually vertically onto the substrate surface, and promote the chemical reaction of the reactive plasma species with the silicon on the etching base.
Because of the nearly vertical fall of the cations, etching should progress slowly toward the sidewalls of the structures, and in the optimum case, not at all.
It is known to use non-dangerous and process-stable reactive gases based on fluorochemicals. However, although these reactive gases permit a very high etching rate and a very high selectivity between the substrate to be etched and the mask, they display a markedly isotropic etching behavior.
The fluorine radicals generated in the plasma exhibit a high spontaneous reaction rate wherein the structure edges (lateral surfaces) are etched quickly, thus resulting in undesired undercutting of the mask edges, trench sidewalls and undercutting of trenches on the insulator interface.
Different suggestions have been proposed to overcome the problem of undercutting. One such method involves providing a protective layer, for example, in U.S. Pat. No. 4,528,066, a reactive etching technique is disclosed for etching a gate electrode out of layers of tungsten silicide and polycrystalline silicon without etching the insulator layer of silicon dioxide. The sidewalls of the gate are protected from etching by application of a layer of polytetrafluoroethylene.
In U.S. Pat. No. 5,501,893, hereafter called the Bosch process, an etching process is disclosed wherein a silicon substrate first undergoes a plasma etching step which is followed by a second polymerizing step wherein exposed areas are covered by a polymer layer which forms a temporary etching stop. These two steps make up the process by alternately repeating the etching step and the polymerizing step.
An alternative method, as detailed herein, involves covering of the side walls with one or more polymer forming compounds, which compounds are present in the plasma, at the same time during etching, thereby protecting the walls with the polymer film. Because the polymer film would also form on the etching base, a stable fall of ions should keep this film free from polymer and permit etching there. However, associated with this technique there is a disadvantage that the polymer forming compounds added to the plasma, which are partly formed from the fluorine carrier itself or through the splitting of fluorine radicals, or which result from purposely added, unsaturated compounds or eroded. Organic photoresist mask material, occur as recombination partners with respect to the fluorine radicals. By means of this back reaction, the objective of which is a chemical equilibrium, a considerable portion of the fluorine required for etching is consumed, while at the same time a corresponding component of the polymer formers required for side wall passivation is lost. Because of this, the etching rate that can be attained with this method is markedly reduced.
This dependence of the etching fluorine radicals on the unsaturated polymer forming compounds in the plasma makes the etching rates and the etching profiles dependent on the free silicon substrate surface to be etched. This is such because the fluorine radicals react with the polymer forming compounds present in the plasma, thereby reducing the fluorine radicals available for etching the silicon substrate.
Additionally, another disadvantage which can occur is when the unsaturated species present in the plasma that result in the polymer forming compounds, preferably etch certain mask materials and thus cause the selectivity, that is, the ratio of the silicon etching rate to the mask etching rate, to worsen. Furthermore, if a non-uniform side wall protection occurs, the side walls are more heavily coated directly at the mask edge with polymer, and thus the side wall is better protected in this area than in the progressive etching depth of the structures.
If this is the case, the polymer covering of the side walls decreases rapidly at greater depths, and an undercut occurs there with the consequence that bottle-type etching profiles result.
Instead of using reactive gases based on fluorine, it has been proposed to use reactive gases based on other halogens, such as chlorine and bromine or reactive gases that release chlorine or bromine in plasma, because these gases are less reactive on the silicon surface.
The radicals from the reactive gases, typically fluorine radicals generated from, e.g., SF.sub.6, C.sub.4 F.sub.8, and NF.sub.3, formed in the plasma exhibit a significantly higher spontaneous reaction with silicon, and first lead to etching with simultaneous ion support. It is generally known that in a capacitively coupled RF lower electrode, a negative self induced DC bias potential is developed on the electrode with respect to ground. Thus, these reactive gases offer the advantage of essentially etching only on the bottom of the structure, and not on the side walls of the structure because the ions impact virtually vertically on the silicon substrate. The disadvantage exists, however, that these reactive gases react in an extraordinarily sensitive manner with respect to moisture.
In this case, not only are costly transfer devices necessary for the silicon substrates in the reactor, but also the leakage rate of the entire etching system must be kept extremely low. Even the slightest occurrence of reactor moisture leads to microroughness on the bottom of the silicon etching due to local silicon oxidation, and thus to a complete breakdown in etching.